Method of fabricating semiconductor device

ABSTRACT

In a method of fabricating a semiconductor device having a MISFET of trench gate structure, a trench is formed from a major surface of a semiconductor layer of first conductivity type which serves as a drain region, in a depth direction of the semiconductor layer, a gate insulating film including a thermal oxide film and a deposited film is formed over the internal surface of the trench, and after a gate electrode has been formed in the trench, impurities are introduced into the semiconductor substrate of first conductivity type to form a semiconductor region of second conductivity type which serves as a channel forming region, and impurities are introduced into the semiconductor region of second conductivity type to form the semiconductor region of first conductivity type which serves as a source region.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device and, moreparticularly, to an art which can be usefully applied to a semiconductordevice having a transistor element of trench gate structure.

BACKGROUND OF THE INVENTION

Power transistors (semiconductor devices) are used as switching elementsfor power amplification circuits, power supply circuits and the like.This kind of power transistors has a construction in which a pluralityof transistor elements are electrically connected in parallel. Each ofthe transistor elements is constructed as, for example, a MISFET (MetalInsulator Semiconductor Field Effect Transistor) of trench gatestructure. A method of fabricating a power transistor having a MISFET oftrench gate structure will be described below.

First, an n⁻-type semiconductor layer is formed over a major surface ofan n⁺-type semiconductor substrate made of single-crystal silicon by anepitaxial growth method. These n⁺-type semiconductor substrate andn⁻-type semiconductor layer are used as a drain region. Then, p-typeimpurities are introduced into the entire major surface of the n⁻-typesemiconductor layer by ion implantation to form a p-type semiconductorregion to be used as a channel forming region. Then, n-type impuritiesare selectively introduced into the major surface of the p-typesemiconductor region by ion implantation to form an n⁺-typesemiconductor region which serves as a source region.

Then, after, for example, a silicon oxide film has been formed over themajor surface of the n⁻-type semiconductor layer, patterning is appliedto the silicon oxide film to form a mask having an opening above atrench forming region of the n⁻-type semiconductor layer. Then, a trenchis formed from the major surface of the n⁻-type semiconductor layer inthe depth direction thereof by using the mask as an etching mask. Theformation of the trench is performed by an anisotropic dry etchingmethod.

Then, wet etching is applied to allow the mask to recede from the topedge portion of the trench (the portion of intersection of the sidesurface of the trench and the major surface of the n⁻-type semiconductorlayer). Then, isotropic dry etching is applied to form the top edgeportion and the bottom edge portion.(the portion of intersection of theside surface of the trench and the bottom surface thereof) of the trenchinto gently-sloping shapes, respectively. Then, the mask is removed.

Then, thermal oxidation is applied to form a sacrifice thermal oxidefilm over the internal surface of the trench, and then the sacrificethermal oxide film is removed. The formation and the removal of thesacrifice thermal oxide film are performed for the purpose of removingdefects, strain, contamination and the like which are produced when thetrench is formed.

Then, thermal oxidation is applied to form a gate insulating filmcomprising a thermal oxide film over the internal surface of the trench.Then, a polycrystalline silicon film is formed over the entire majorsurface of the n⁻-type semiconductor layer, inclusive of the inside ofthe trench, by a chemical vapor deposition method. Impurities fordecreasing the resistance value of the polycrystalline silicon film areintroduced into the polycrystalline silicon film during or after thedeposition thereof.

Then, etchback is applied to flatten the surface of the polycrystallinesilicon film. Then, etching is selectively applied to thepolycrystalline silicon film to form a gate electrode in the trench andto form a gate lead-out electrode integrated with the gate electrode,over the peripheral region of the major surface of the n⁻-typesemiconductor layer. In this step, a MISFET is formed which has a trenchgate structure in which the gate electrode is formed in the trench ofthe n⁻-type semiconductor layer, with the gate insulating filminterposed therebetween

Then, an interlayer insulating film is formed over the entire majorsurface of the n⁻-type semiconductor layer, inclusive of the top surfaceof the gate electrode, and then a contact hole is formed in theinterlayer insulation film. After that, a source interconnection and agate interconnection are formed, and then a final passivation film isformed. After that, a bonding opening is formed in the final passivationfilm, and then a drain electrode is formed on the back of the n⁺-typesemiconductor substrate, whereby a power transistor having such a MISFETof trench gate structure is almost finished.

The MISFET having the trench gate structure constructed in this mannercan be reduced in its occupation area compared to a MISFET in which itsgate electrode is formed on the major surface of its semiconductorlayer, with a gate insulating film interposed therebetween. Accordingly,the size and on resistance of the power transistor can be reduced.

Incidentally, a power transistor having a MISFET of trench gatestructure is described in, for example, EP 666,590.

SUMMARY OF THE INVENTION

The present inventors have examined the above-described power transistor(semiconductor device) and found out the following problems.

In the case of the above-described power transistor, the p-typesemiconductor region which serves as the channel forming region isformed in the n⁻-type semiconductor layer which serves as the drainregion, and the n⁺-type semiconductor region which serves as the sourceregion is formed in the p-type semiconductor region, and after thetrench has been formed in the n⁻-type semiconductor layer, thermaloxidation is applied to form the thermal oxide film which serves as thegate insulating film, over the internal surface of the trench.Therefore, impurities in the p-type semiconductor region (for example,boron (B)) or impurities of the n⁺-type semiconductor region (forexample, arsenic (As)) is introduced into the thermal oxide film and thebreakdown voltage of the gate insulating film becomes easily degraded,so that the reliability of the power transistor lowers.

Further, impurities in the p-type semiconductor region at the sidesurface of the trench migrate into the thermal oxide film and avariation occurs in the impurity concentration in the channel formingregion at the side surface of the trench, so that a variation occurs inthe threshold voltage (Vth) of the MISFET and FET characteristics cannotbe provided stably with good reproducibilty.

In addition, impurities of the n⁺-type semiconductor region which servesas the source region undergo enhanced diffusion by the thermal treatmenttemperature during the formation of the thermal oxide film, and theeffective channel length of the MISFET is shortened and thepunch-through breakdown voltage thereof is lowered. If the thermal oxidefilm is formed at a low thermal treatment temperature of approximately950° C., enhanced diffusion of impurities in the n⁺-type semiconductorsubstrate which serves as the source region can be suppressed and thepunch-through breakdown voltage of the MISFET can be ensured. However,if the thermal oxide film is formed at such a low thermal treatmenttemperature, the top edge portion of the trench is deformed into anangular shape by a compressive stress produced during the growth of thethermal oxide film, the film thickness of the thermal oxide film at thetop edge portion becomes locally thin, so that the gate breakdownvoltage of the MISFET is lowered. Therefore, if the thermal oxide filmis formed at a high thermal treatment temperature of approximately1,100° C., the deformation of the top edge portion of the trench can besuppressed and the gate breakdown voltage of the MISFET can be ensured.However, if the thermal oxide film is formed at a high thermal treatmenttemperature of approximately 1,100° C., as described above, impuritiesin the n⁺-type semiconductor substrate which serves as the source regionundergo enhanced diffusion and the punch-through breakdown voltage ofthe MISFET is lowered. In other words, since neither the punch-throughbreakdown voltage nor the gate breakdown voltage of the MISFET can beensured, the reliability of the power transistor is lowered.

An object of the present invention is to provide an art capable ofincreasing the reliability of a semiconductor device and providingstable FET characteristics of good reproducibility.

The above and other objects and novel features of the present inventionwill become apparent from the following description taken in conjunctionwith the accompanying drawings.

Representative aspects of the invention disclosed herein will bedescribed below in brief.

In a method of fabricating a semiconductor device having a MISFET oftrench gate structure, a trench is formed from a major surface of asemiconductor layer of first conductivity type which serves as a drainregion, in the depth direction of the semiconductor layer, a gateinsulating film comprising a thermal oxide film and a deposition film isformed over the internal surface of the trench, and after a gateelectrode has been formed in the trench, impurities are introduced intothe semiconductor substrate of first conductivity type to form asemiconductor region of second conductivity type which serves as achannel forming region, and impurities are introduced into the secondconductivity type semiconductor region to form a semiconductor region offirst conductivity type which serves as a source region. The formationof the thermal oxide film is performed in an oxygen gas atmosphere or ina water vapor atmosphere, and the formation of the deposition film isperformed with a chemical vapor deposition method. The deposited film isa silicon oxide film or a silicon nitride film or an acid nitride film.

According to the above-described means, after the thermal oxide filmwhich serves as the gate insulating film has been formed, thesemiconductor region of second conductivity type which serves as thechannel forming region and the semiconductor region of firstconductivity type which serves as the source region are formed.Accordingly, neither impurities in the semiconductor region of secondconductivity type nor the impurity of the of semiconductor region offirst conductivity type is introduced into the thermal oxide film, andthe degradation of the breakdown voltage of the gate insulating film dueto the introduction of such impurities can be suppressed. Inconsequence, the reliability of the semiconductor device can beimproved.

In addition, since the semiconductor region of first conductivity typewhich serves as the channel forming region is formed after the thermaloxide film which serves as the gate insulating film has been formed,impurities in the semiconductor region of second conductivity type atthe side surface of the trench is not introduced into the thermal oxidefilm, and variation in the threshold voltage (Vth) of the MISFET due tothe variation of the impurity concentration of the channel formingregion can be suppressed. In consequence, stable FET characteristics canbe obtained with good reproducibility.

In addition, since the semiconductor region of first conductivity typewhich serves as the source region is formed after the thermal oxide filmwhich serves as the gate insulating film has been formed, even ifthe-formation of the thermal oxide film is performed at a high thermaloxidation temperature of approximately 1,100° C., impurities in thesemiconductor region of first conductivity type undergo enhanceddiffusion, whereby reduction in effective-channel length can besuppressed and the punch-through breakdown voltage of the MISFET can beensured. In addition, the formation of the thermal oxide film isperformed at a low thermal oxidation temperature of approximately 950°C., and even if the top edge portion of the trench (the portion ofintersection of the side surface of the trench and the major surface ofthe semiconductor layer of first conductivity type) is deformed into anangular shape by a compressive stress produced during the growth of thethermal oxide film, and the film thickness of the thermal oxide film atthe top edge portion becomes locally thin, the locally thin portion canbe compensated for by the deposited film, and therefore the gatebreakdown voltage of the MISFET can be ensured. In consequence, it ispossible to improve the reliability of the power transistor(semiconductor device).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the essential portion of a powertransistor (semiconductor device) of a first embodiment according to thepresent invention;

FIG. 2 is a cross-sectional view taken along the line A-A shown in FIG.1;

FIG. 3 is a cross-sectional view taken along the line B-B shown in FIG.1;

FIG. 4 is a cross-sectional view for illustrating a method offabricating the power transistor;

FIG. 5 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 6 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 7 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 8 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 9 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 10 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 11 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 12 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 13 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 14 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 15 is a cross-sectional view for illustrating a method offabricating a power transistor of a second embodiment according to thepresent invention;

FIG. 16 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 17 is a schematic cross-sectional view for illustrating the methodof fabricating the power transistor;

FIG. 18 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 19 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 20 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 21 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 22 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 23 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 24 is a cross-sectional view for illustrating the method offabricating the power transistor;

FIG. 25 is a cross-sectional view for illustrating the method offabricating the power transistor; and

FIG. 26 is a cross-sectional view for illustrating the method offabricating the power transistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowin detail with reference to the accompanying drawings. Throughout allthe drawings for illustrating the preferred embodiments of the presentinvention, identical reference numerals denote constituent portionshaving identical functions, and the repetition of the same descriptionwill be omitted.

First Embodiment

FIG. 1 is a plan view showing the essential portion of a powertransistor (semiconductor device) of a first embodiment according to thepresent invention. FIG. 2 is a cross-sectional view taken along the lineA-A shown in FIG. 1, and FIG. 3 is a cross-sectional view taken alongthe line B-B shown in FIG. 1. In FIG. 1, a source interconnection 12A, agate interconnection 12B, a final passivation film 13 and the like, allof which will be described later, are not shown for the sake ofsimplicity of illustration. In FIGS. 2 and 3, hatching (slant lines)indicative of a cross section is partly omitted for the sake ofsimplicity of illustration.

As shown in FIGS. 1 and 2, the power transistor of the first embodimentincludes as its principal body a semiconductor base in which, forexample, an n⁻-type semiconductor layer 1B is formed over a majorsurface of an n⁺-type semiconductor substrate 1A made of single-crystalsilicon. The n⁻-type semiconductor layer 1B is formed by, for example,an epitaxial growth method, and is made of single-crystal silicon.

A plurality of transistor elements are formed in the semiconductor base,and are electrically connected in parallel. The transistor elements ofthe first embodiment are MISFETs.

Each of the MISFETs principally includes a channel forming region, agate insulating film 5, a gate electrode 6A, a source region and a drainregion. The channel forming region comprises a p-type semiconductorregion 8 formed in an n⁻-type semiconductor layer 1B. The source regioncomprises an n⁺-type semiconductor region 9 formed in the p-typesemiconductor region 8. The drain region comprises an n⁺-typesemiconductor substrate 1A and the n⁻-type semiconductor layer 1B. Thegate insulating film 5 is formed on the internal surface of a trench 4which is formed from the major surface of the n⁻-type semiconductorlayer 1B in the depth direction thereof. The gate electrode 6A comprisesa conductive film buried in the trench 4, with the gate insulating film5 interposed therebetween. The conductive film comprises, for example, apolycrystalline silicon film in which impurities for decreasing theresistance value is introduced. In other words, the MISFET has avertical structure in which the source region, the channel formingregion and the drain region are disposed in that order from the majorsurface of the n⁻-type semiconductor layer 1B in the depth directionthereof, and further has a trench gate structure in which the gateinsulating film 5 and the gate electrode 6A are formed in the trench 4formed in the n⁻-type semiconductor layer 1B. In addition, the MISFET isof an n-channel conductivity type in which the p-type semiconductorregion 8 at side surface of the trench 4 is used as the channel formingregion.

The gate insulating film 5 of the MISFET is, but not limited to, amultilayer film in which, for example, a thermal oxide film 5A and adeposited film 5B are disposed in that order from the internal surfaceof the trench 4. The thermal oxide film 5A is formed with a filmthickness of, for example, approximately 20 nm, and the deposited film5B is formed with a film thickness of, for example, approximately 50 nm.The thermal oxide film 5A is formed by forming the trench 4 in then⁻-type semiconductor layer 1B and then conducting a heat treatment ofapproximately 950° C. in an oxygen gas atmosphere or a water vaporatmosphere. The deposited film 5B is a silicon oxide film deposited by,for example, a chemical vapor deposition method. This silicon oxide filmis formed by causing silane (SiH₄) to react with oxygen (O₂) in anatmosphere with a temperature of, for example, approximately 800° C.

The element forming region of the major surface of the n⁻-typesemiconductor layer 1B is divided into a plurality of island regions bythe trench 4. These island regions are regularly disposed in a matrix,and each of the island regions has a flat octagonal shape in plan view.In other words, the trench 4 is formed in such a pattern that theelement forming region of the major surface of the n⁻-type semiconductorlayer 1B is divided into the plurality of island regions and each ofthese island regions has the flat octagonal shape in plan view.Incidentally, the n⁺-type semiconductor region 9 which serves as thesource region of the MISFETs is formed over the major surface of each ofthe island regions into which the element forming region of the n⁻-typesemiconductor layer 1B is divided by the trench 4.

The top edge portion of the trench 4 (the portion of intersection of theside surface of the trench 4 and the major surface of the n⁻-typesemiconductor layer 1B) and the bottom edge portion of the trench A (theportion of intersection of the side surface of the trench 4 and thebottom surface thereof) have gently-sloping shapes. The shapes of thetop edge portion and the bottom edge portion of the trench 4 are formedby forming the trench 4 in the n⁻-type semiconductor layer 1B and thenapplying chemical dry. etching using a mixture gas of a chlorine gas andan oxygen gas.

The source interconnection 12A is electrically connected to each of then⁺-type semiconductor region 9 and the p-type semiconductor region 8through a contact hole 11A formed in an interlayer insulation film 10.The interlayer insulation film 10 is provided between the gate electrode6A and the source interconnection 12A and electrically isolates the gateelectrode 6A and the source interconnection 12A from each other. Thesource interconnection 12A is, for example, an aluminum (Al) film or analuminum alloy film. An insulating film 7 is provided between the gateelectrode 6A and the interlayer insulation film 10.

As shown in FIGS. 1 and 3, the gate electrode 6A is extended to aperipheral region of the major surface of the n-type semiconductor layer1, and is integrated with a gate lead-out electrode 6B formed over themajor surface in the peripheral region. The gate interconnection 12B iselectrically connected to the gate lead-out electrode 6B through acontact hole 11B formed in the interlayer insulation film 10. The gateinterconnection 12B is formed in the same layer as the sourceinterconnection 12A, and is electrically isolated therefrom.

As shown in FIGS. 2 and 3, a final passivation film 13 is formed overthe entire major surface of the n⁻-type semiconductor layer 1B,inclusive of the top surface of the source interconnection 12A and thetop surface of the gate interconnection 12B. The final passivation film13 is, for example, a silicon oxide film deposited by a plasma chemicalvapor deposition method which uses a tetraethoxysilane (TEOS) gas as theprincipal component of a source gas. Incidentally, a bonding opening inwhich the surface of the source interconnection 12A is partly exposed isformed in the final passivation film 13, and further, a bonding openingin which the surface of the gate interconnection 12B is partly exposedis formed in the final passivation film 13.

A drain electrode 14 is formed over the back of the n-type semiconductorlayer 1.

A method of fabricating the above-described power transistor will bedescribed below with reference to FIGS. 4 to 14 (which arecross-sectional views for illustrating the method of fabricating thepower transistor). Throughout FIGS. 8 to 14, hatching (slant lines)indicative of a cross section is partly omitted for the sake ofsimplicity of illustration.

First, the n⁺-type semiconductor substrate 1A made of single-crystalsilicon is prepared. The impurity concentration of the n⁺-typesemiconductor substrate 1A is set to approximately 2×10¹⁹ atoms/cm³. Forexample, arsenic (As) is introduced as impurities.

Then, as shown in FIG. 4, the n⁻-type semiconductor layer 1B is formedover the major surface of the n⁺-type semiconductor substrate 1A by anepitaxial growth method. The n⁻-type semiconductor layer 1B is so formedas to have, for example, a resistivity value of approximately 0.4 Ωcmand a thickness of-approximately 6 μm. In this step, the n-typesemiconductor layer 1 which includes the n⁺-type semiconductor substrate1A and the n⁻-type semiconductor layer 1B is formed.

Then, a silicon oxide film having a film thickness of approximately 500nm is formed over the major surface of the n⁻-type semiconductor layer1B. This silicon oxide film is formed by, for example, a thermaloxidation method.

Then, patterning is applied to the silicon oxide film to form a mask 2having an opening 3 above a trench forming region of the n⁻-typesemiconductor layer 1B, as shown in FIG. 5. This mask 2 is formed in apattern in which each region defined by the opening 3 has a flatoctagonal shape in plan view in the element forming region of the majorsurface of the n⁻-type semiconductor layer 1B.

Then, the trench 4 is formed from the major surface of the n⁻-typesemiconductor layer 1B in the depth direction thereof as shown in FIG.6, by using the mask 2 as an etching mask. The formation of the trench 4is performed by an anisotropic dry etching method which uses, forexample, a chlorine gas or a hydrogen bromide gas and RF (RadioFrequency) power of high level. The trench 4 is so formed as to have adepth of approximately 1.5-2 μm and a width of approximately 0.5-2 μm.

Then, wet etching is applied to allow the mask 2 to recede byapproximately 200 nm from the top edge portion of the trench 4 (theportion of intersection of the side surface of the trench 4 and themajor surface of the n⁻-type semiconductor layer 1B).

Then, as shown in FIG. 7, chemical dry etching using a mixture gas of achlorine gas and an oxygen gas is applied to form the top edge portionand the bottom edge portion (the portion of intersection of the sidesurface and the bottom surface of the trench 4) of the trench 4 intogently-sloping (rounded) shapes. In this step, the trench 4 having thetop and bottom edge portions of gently-sloping shapes is obtained. Afterthat, the mask 2 is removed.

Then, thermal oxidation is applied to form a sacrifice thermal oxidefilm having a film thickness of approximately 100 nm over the internalsurface of the trench 4, and then the sacrifice thermal oxide film isremoved. The formation and the removal of the sacrifice thermal oxidefilm are performed for the purpose of removing defects, strain,contamination and the like produced when the trench 4 is formed. Theformation of the sacrifice thermal oxide film is performed in an oxygengas atmosphere at a high temperature of approximately 1,100° C. If theformation of the sacrifice thermal oxide film is performed at a lowthermal oxidation temperature of approximately 950° C., the top edgeportion of the trench 4 which has been formed into a gently-slopingshape in the previous step will be deformed into an angular shape by acompressive stress produced during the growth of the sacrifice thermaloxide film. The reason is that the formation of the sacrifice thermaloxide film is performed at a thermal oxidation temperature of 1,000° C.or more. Incidentally, the formation of the sacrifice thermal oxide filmmay also be performed in an oxygen gas atmosphere diluted with anitrogen gas.

Then, thermal oxidation is applied to form the thermal oxide film 5Ahaving a film thickness of approximately 20 nm over the internal surfaceof the trench 4 as shown in FIG. 8. After that, as shown in FIG. 9, thedeposition film 5B made of silicon oxide and having a film thickness ofapproximately 50 nm is deposited over the surface of the thermal oxidefilm 5A by a chemical vapor deposition method, thereby forming the gateinsulating film 5. The formation of the thermal oxide film 5A isperformed in an oxygen gas atmosphere or a water vapor atmosphere havinga low temperature of approximately 950° C. The deposition of thedeposition film 5B is performed in a temperature atmosphere having a lowtemperature of approximately 950° C. In this step of forming the gateinsulating film 5, since the formation of the thermal oxide film 5A isperformed at a low thermal oxidation temperature of approximately 950°C., the top edge portion of the trench 4 (the portion of intersection ofthe side surface of the trench 4 and the major surface of the n⁻-typesemiconductor layer 1B) which has been formed into a gently-slopingshape in the previous step is deformed into an angular shape by acompressive stress produced during the growth of the thermal oxide film5A and the film thickness of the thermal oxide film 5A at the top edgeportion becomes locally thin. However, since that locally thin portionis compensated for by the deposition film 5B, the breakdown voltage ofthe gate insulating film 5 is ensured.

Then, for example, a polycrystalline silicon film is formed as aconductive film over the entire major surface of the n⁻-typesemiconductor layer 1B inclusive of the inside of the trench 4, by achemical vapor deposition method. Impurities for decreasing theresistance value (for example, phosphorus (P)) are introduced into thepolycrystalline silicon film during or after the deposition thereof. Thepolycrystalline silicon film is so formed as to have a film thicknessof, for example, approximately 1 μm.

The surface of the polycrystalline silicon is flattened. This flatteningis performed by, for example, an etchback method or a chemicalmechanical polishing (CMP) method.

Then, etching is selectively applied to the polycrystalline silicon filmto form the gate electrode 6A in the trench 4 as shown in. FIG. 10 andto form the gate lead-out electrode 6B (shown in FIG. 3) integrated withthe gate electrode 6A, over the peripheral region of the major surfaceof the n⁻-type semiconductor layer 1B.

Then, after the deposition film 5B and the thermal oxide film 5A whichremain on the major surface of the n⁻-type semiconductor layer 1B havebeen removed, the insulating film 7 made of, for example, silicon oxideis formed over the entire major surface of the n⁺-type semiconductorlayer 1B, inclusive of the top surface of the gate electrode 6A and thetop surface of the gate lead-out electrode 6B, as shown in FIG. 11. Theformation of the insulating film 7 is performed by a thermal oxidationmethod or a chemical vapor deposition method.

Then, after a p-type impurity (for example, boron) has been introducedinto the entire major surface of the n⁻-type semiconductor layer 1B byion implantation, stretched diffusion process is conducted to form thep-type semiconductor region 8 which serves as the channel formingregion, as shown in FIG. 11. The stretched diffusion process isperformed for about one hour in an N₂ gas atmosphere having atemperature of approximately 1,100° C.

Then, after an n-type impurity (for example, arsenic) has beenselectively introduced into a major surface of the p-type semiconductorregion 8 which constitutes the major surface of the n⁻-typesemiconductor layer 1B, by ion implantation, annealing is performed at atemperature of 950° C. for about 20 minutes to form the n⁺-typesemiconductor region 9 which serves as a source region, as shown in FIG.12. The introduction of the n-type impurity is performed under thecondition that the amount of n-type impurity to be finally introduced isset to approximately 5×10¹⁵ atoms/cm² and the amount of energy requiredduring the introduction is set to 80 KeV. In this step, a MISFET isformed which has a trench gate structure in which the gate insulatingfilm 5 and the gate electrode 6A are formed in the trench 4 of then⁻-type semiconductor layer 1B.

In the above-described steps, the p-type semiconductor region 8 whichserves as the channel forming region and the n⁺-type semiconductorsubstrate 9 which serves as the source region are formed after thethermal oxide film SA which constitutes the gate insulating film 5 hasbeen formed. Accordingly, in the step of forming the thermal oxide film5A, neither impurities in the p-type semiconductor region 8 norimpurities in the n⁺-type semiconductor region 9 do not migrate into thethermal oxide film 5A, and therefore it is possible to suppressdegradation of the breakdown voltage of the gate insulating film 5 dueto the migration of impurities.

The p-type semiconductor region 8 which serves as the channel formingregion is formed after the thermal oxide film 5A which constitutes thegate insulating film 5 has been formed. Accordingly, impurities in thep-type semiconductor region 8 at the side surface of the trench 4 do notmigrate into the thermal oxide film 5A, and therefore it is possible tosuppress the variation in the threshold voltage (Vth) of the MISFET dueto the variation of the impurity concentration in the channel formingregion.

The n⁺-type semiconductor region 9 which serves as the source region isformed after the thermal oxide film 5A which constitutes the gateinsulating film 5 has been formed. Accordingly, even if the formation ofthe thermal oxide film 5A is performed at a high thermal oxidationtemperature of approximately 1,100° C., impurities in the n⁺-typesemiconductor region 9 do not undergo enhanced diffusion wherebyreduction in effective channel length can be suppressed and thepunch-through breakdown voltage of the MISFET can be ensured. Inaddition, the-formation of the thermal oxide film 5A is performed at alow thermal oxidation temperature of approximately 950° C., and even ifthe top edge portion of the trench 4 (the portion of intersection of theside surface of the trench 4 and the major surface of the n⁻-typesemiconductor layer 1B.) is deformed into an angular shape by acompressive stress produced during the growth of the thermal oxide film5A and the film thickness of the thermal oxide film 5A at the top edgeportion becomes locally thin, that locally thin portion can becompensated for by the deposition film 5B, and therefore the gatebreakdown voltage of the MISFET can be ensured.

Then, as shown in FIG. 13, the interlayer insulation film 10 having afilm thickness of, for example, approximately 500 nm is formed over theentire surface of the n⁻-type semiconductor layer 1B. The interlayerinsulation film 10 is, for example, a BPSG (Boro Phospho Silicate Glass)film.

Then, anisotropic dry etching using CHF₃ gas is performed to form thecontact hole 11A and the contact hole 11B (shown in FIG. 3) in theinterlayer insulation film 10, as shown in FIG. 14.

Then, after a conductive film comprising, for example, an aluminum filmor an aluminum alloy film has been formed over the entire major surfaceof the n⁻-type semiconductor layer 1B, inclusive of the insides of thecontact holes 11A and 11B, patterning is applied to the conductive filmto form the source interconnection 12A to be electrically connected toeach of the p-type semiconductor region 8 and the n⁺-type semiconductorregion 9, and-to form the gate interconnection 12B to be electricallyconnected to the gate lead-out electrode 6B.

Then, the final passivation film 13 is formed over the entire majorsurface of the n⁻-type semiconductor layer 1B, inclusive of the topsurface of the source interconnection 12A and the top surface of thegate lead-out electrode 6B. The final passivation film 13 is, forexample, a silicon oxide film deposited by a plasma chemical vapordeposition method which uses a tetraethoxysilane (TEOS) gas as aprincipal component of a source gas.

Then, a bonding opening in which the surface of a part of the sourceinterconnection 12A is exposed and a bonding opening in which thesurface of a part of the gate interconnection 12B is exposed are formedin the final passivation film 13. After that, the back of the n⁺-typesemiconductor substrate 1A is ground, and then the drain electrode 14 isformed on the back of the n⁺-type semiconductor substrate 1A. Thus, thepower transistor having a MISFET having the trench gate structure isalmost finished.

As is apparent from the above description, the first embodiment has thefollowing effects.

The first embodiment is a method of fabricating a semiconductor devicehaving a MIS-FET of trench gate structure, which comprises the steps offorming a trench 4 from the surface of an n⁻-type semiconductor layer 1Bwhich serves as a drain region, in the depth direction of an n⁻-typesemiconductor layer 1B, forming a gate insulating film 5 comprising athermal oxide film 5A and a deposition film 5B over the internal surfaceof the trench 4, forming a gate electrode 6A in the trench 4,introducing impurities into the n⁻-type semiconductor layer 1B to form ap-type semiconductor region 8 which serves as a channel forming region,and introducing impurities into the p-type semiconductor region 8 toform an n⁺-type semiconductor region 9 which serves as a source region.

In this method, after the thermal oxide film 5A which constitutes thegate insulating film 5 has been formed, the p-type semiconductor region8 which serves as the channel forming region and the n⁺-typesemiconductor region 9 which serves as the source region are formed.Accordingly, neither impurities in the p-type semiconductor region 8 norimpurities in the n⁺-type semiconductor region 9 migrate into thethermal oxide film 5A, and therefore it is possible to suppressdegradation of the breakdown voltage of the gate insulating film 5 dueto the introduction of impurities. In consequence, it is possible toimprove the reliability of the power transistor (semiconductor device).

In addition, since the p-type semiconductor region 8 which serves as thechannel forming region is formed after the thermal oxide film 5A whichconstitutes the gate insulating film 5 has been formed, impurities inthe p-type semiconductor region 8 which at the side surface of thetrench 4 do not migrate into the thermal oxide film 5A, and therefore itis possible to suppress the variation in the threshold voltage (Vth) ofthe MISFET due to the variation of the impurity concentration in thechannel forming region. In consequence, it is possible to obtain stableFET characteristics with good reproducibility.

In addition, since the n⁺-type semiconductor region 9 which serves asthe source region is formed after the thermal oxide film 5A whichconstitutes the gate insulating film 5 has been formed, even if theformation of the thermal oxide film 5A is performed at a high thermaloxidation temperature of approximately 1,100° C., impurities in then⁺-type semiconductor region 9 do not undergo enhanced diffusion,whereby reduction in effective channel length can be suppressed and thepunch-through breakdown voltage of the MISFET can be ensured. Inaddition, the formation of the thermal oxide film 5A is performed at alow thermal oxidation temperature of approximately 950° C., and even ifthe top edge portion of the trench 4 (the portion of intersection of theside surface of the trench 4 and the major surface of the n⁻-typesemiconductor layer 1B) is deformed into an angular shape by acompressive stress produced during the growth of the thermal oxide film5A and the film thickness of the thermal oxide film 5A at the top edgeportion becomes locally thin, that locally thin portion can becompensated for by the deposition film 5B, and therefore the gatebreakdown voltage of the MISFET can be ensured. In consequence, it ispossible to improve the reliability of the power transistor(semiconductor device).

Incidentally, although the first embodiment has been described withreference to the example in which the deposition film 5B comprises asilicon oxide film, the deposition film 5B may also be a silicon nitridefilm or an acid nitride film.

Second Embodiment

A second embodiment will be described, taking an example in which a maskto be used as an etching mask during the formation of a trench is amultilayer film including a silicon oxide film, a silicon nitride filmand a silicon oxide film. The reason why the mask is such multilayerfilm is that if the mask to be used as an etching mask during theformation of a trench is a single-layer film of silicon oxide as in thefirst embodiment, a hydrofluoric acid-containing etchant needs to beused for removing a reactive deposit produced during anisotropic etchingand, at this time, if the film thickness of the mask 2 shown in FIG. 6is excessively thin, the mask 2 is removed after the etching, and theprocess of forming the top edge portion of the trench into agently-sloping shape by isotropic etching cannot be carried out.

In addition, under particular conditions of anisotropic etching, sincereactive deposit is produced as a thin layer over the side surface ofthe trench, it is necessary to carry out etching using hydrofluoric acidor the like for a long time in order to remove the reactive deposited,so that there is a good possibility that a mask is absent duringisotropic etching for forming the top edge portion of the trench into agently-sloping shape. In the second embodiment, after the trench hasbeen formed, it is possible to effect satisfactory etching using ahydrofluoric acid etchant and the like by using a silicon nitride(Si₃N₄) film, which is not at all etched by a hydrofluoricacid-containing etchant, as a mask material during trench formation.Accordingly, since a silicon oxide film which is a film underlying thesilicon nitride film can be preserved even after isotropic etching, thetop edge portion of the trench can be formed into a gently-slopingshape.

A method of fabricating a power transistor of the second embodimentaccording to the present invention will be described below withreference to FIGS. 15 to 26. Throughout FIGS. 19 to 26, hatching (slantlines) indicative of a cross section is partly omitted for the sake ofsimplicity of illustration.

First, the n⁻-type semiconductor layer 1B is formed over the majorsurface of the n⁺-type semiconductor substrate 1A made of single-crystalsilicon, by an epitaxial growth method. The n⁻-type semiconductor layer1B is so formed as to have, for example, a resistivity value ofapproximately 0.4 Ωcm and a thickness of approximately 6 μm. In thisstep, a semiconductor base which includes the n⁺-type semiconductorsubstrate 1A and the n⁻-type semiconductor layer 1B is formed.

Then, as shown in FIG. 15, a silicon oxide film 2A having a filmthickness of approximately 100 nm, a silicon nitride film 2B having afilm thickness of approximately 200 nm and a silicon oxide film 2Chaving a film thickness of approximately 400 nm are formed in that orderover the major surface of the n⁻-type semiconductor layer 1B. Thesilicon oxide film 2A is formed by a thermal oxidation method, and thesilicon nitride film 2B and the silicon oxide film 2C are formed by achemical vapor deposition method.

Then, patterning is applied to the silicon oxide film 2C, the siliconnitride film 2B and the silicon oxide film 2A in that order byanisotropic dry etching using a gas such as CHF₃, thereby forming themask 2 having the opening 3 above a trench forming region of the n⁻-typesemiconductor layer 1B as show in FIG. 16.

Then, the trench 4 is formed from the major surface of the n⁻-typesemiconductor layer 1B in the depth direction thereof as shown in FIG.17, by using the mask 2 as an etching mask. The formation of the trench4 is performed by an anisotropic dry etching method which uses, forexample, a chlorine gas or a hydrogen bromide gas and RF (RadioFrequency) power set to a high level. The trench 4 is so formed as tohave a depth of approximately 1.5-2 μm and a width of approximately0.5-2 μm.

Then, wet etching is performed to allow the silicon oxide film 2A of themask 2 to recede by approximately 500 nm to 1μm from the top edgeportion of the trench 4 (the portion of intersection of the side surfaceof the trench 4 and the major surface of the n⁻-type semiconductor layer1B). In this step, a reactive deposit produced over the side surface ofthe trench 4 and the silicon oxide film 2C are completely removed, andthe surface of the silicon nitride film 2B is exposed.

Then, chemical dry etching using a mixture gas of a chlorine gas and anoxygen gas is performed to form the top edge portion and the bottom edgeportion (the portion of intersection of the side surface and the bottomsurface of the trench 4) of the trench 4 into gently-sloping shapes, asshown in FIG. 18. In this step, the trench 4 having the top and bottomedge portions of gently-sloping shapes is formed.

Then, thermal oxidation is conducted to form a sacrifice thermal oxidefilm having a film thickness of approximately 100 nm over the internalsurface of the trench 4, and then the sacrifice thermal oxide film isremoved. The formation of the sacrifice thermal oxide film is performedin an oxygen gas atmosphere with a high temperature of approximately1,100° C. If the formation of the sacrifice thermal oxide film isperformed at a low thermal oxidation temperature of approximately 950°C., the top edge portion of the trench 4 which has been formed into agently-sloping shape in the previous step is deformed into an angularshape by a compressive stress produced during the growth of thesacrifice thermal oxide film. The reason is that the formation of thesacrifice thermal oxide film is performed at a thermal oxidationtemperature of 1,000° C. or higher. Incidentally, the formation of thesacrifice thermal oxide film may also be performed in an oxygen gasatmosphere diluted with a nitrogen gas.

Then, thermal oxidation is performed to form the thermal oxide film 5Ahaving a film thickness of approximately 20 nm over the internal surfaceof the trench 4 as shown in FIG. 19. After that, as shown in FIG. 20,the deposition film 5B made of silicon oxide having a film thickness ofapproximately 50 nm is deposited over the surface of the thermal oxidefilm 5A by a chemical vapor deposition method, thereby forming the gateinsulating film 5. The formation of the thermal oxide film 5A isperformed in an oxygen gas atmosphere or a water vapor atmosphere of alow temperature of approximately 950° C. The deposition of thedeposition film 5B is performed in a temperature atmosphere having a lowtemperature of approximately 800° C. In this step of forming the gateinsulating film 5, since the formation of the thermal oxide film 5A isperformed at a low thermal oxidation temperature of approximately 950°C., the top edge portion of the trench 4 (the portion of intersection ofthe side surface of the trench 4 and the major surface of the n⁻-typesemiconductor layer 1B) which has been formed into a gently-slopingshape in the previous step is deformed into an angular shape by acompressive stress produced during the growth of the thermal oxide film5A, and the film thickness of the thermal oxide film 5A at the top edgeportion becomes locally thin However, since the locally thin portion iscompensated for by the deposition film 5B, the breakdown voltage of thegate insulating film 5 is ensured.

Then, for example, a polycrystalline silicon film is formed as aconductive film over the entire major surface of the n⁻-typesemiconductor layer 1B, inclusive of the inside of the trench 4, by achemical vapor deposition method. Impurities for decreasing theresistance value (for example, phosphorus) is introduced into thepolycrystalline silicon film during or after the deposition thereof. Thepolycrystalline silicon film is so formed as to have a film thicknessof, for example, approximately 1 μm.

Then, the surface of the polycrystalline silicon is flattened. Thisflattening is performed by, for example, an etchback method or achemical mechanical polishing method.

Then, etching is selectively applied to the polycrystalline silicon filmto form the gate electrode 6A in the trench 4 as shown in FIG. 21 and toform the gate lead-out electrode 6B (shown in FIG. 3) integrated withthe gate electrode 6A, over the peripheral region of the major surfaceof the n⁻-type semiconductor layer 1B.

Then, the deposition film 5B which remains on the major surface of thesilicon nitride film 2B is removed, and further the silicon nitride film2B is removed. After that, as shown in FIG. 22, the insulating film 7made of, for example, silicon oxide is formed over the entire majorsurface of the n⁻-type semiconductor layer 1B, inclusive of the topsurface of the gate electrode 6A and the top surface of the gatelead-out electrode 6B. The formation of the insulating film 7 isperformed by a thermal oxidation method or a chemical vapor depositionmethod.

Then, after a p-type impurity (for example, boron) has been introducedinto the entire major surface of the n⁻-type semiconductor layer 1B byion implantation, stretched diffusion process is performed to form thep-type semiconductor region 8 which serves as the channel formingregion, as shown in FIG. 23. The stretched diffusion process isperformed for about one hour in an N₂ gas atmosphere having atemperature of approximately 1,100° C.

Then, after an n-type impurity (for example, arsenic) has beenselectively introduced into the major surface of the p-typesemiconductor region 8 which is the major surface of the n⁻-typesemiconductor layer 1B, by ion implantation, annealing is performed at atemperature of 950° C. for about 20 minutes to form the n⁺-typesemiconductor region 9 which serves as a source region, as shown in FIG.24. The introduction of the n-type impurity is performed under thecondition that the amount of the n-type impurity to be finallyintroduced is set to approximately 5×10¹⁵ atoms/cm² and the amount ofenergy required during the incorporation is set to 80 KeV. In this step,a MISFET is formed which has a trench gate structure in which the gateinsulating film 5 and the gate electrode 6A are formed in the trench 4of the n⁻-type semiconductor layer 1B. Then, as shown in FIG. 24, theinterlayer insulation film 10 having a film thickness of, for example,approximately 500 nm is formed over the entire surface of the n⁻-typesemiconductor layer 1B. The interlayer insulation film 10 is formed of,for example, a BPSG (Boro Phospho Silicate Glass) film.

Then, anisotropic dry etching using CHF₃ gas is performed to form thecontact hole 11A and the contact hole 11B (shown in FIG. 3) in theinterlayer insulation film 10, as shown in FIG. 25.

Then, after a conductive film comprising, for example, an aluminum filmor an aluminum alloy film has been formed over the entire major surfaceof the n⁻-type semiconductor layer 1B, inclusive of the insides of thecontact holes 11A and 11B, the conductive film is patterned to form thesource interconnection 12A to be electrically connected to the p-typesemiconductor region 8 and the n⁺-type semiconductor region 9, and toform the gate interconnection 12B (shown in FIG. 3) to be electricallyconnected to the gate lead-out electrode 6B.

Then, the final passivation film 13 is formed over the entire majorsurface of the n⁻-type semiconductor layer 1B, inclusive of the topsurface of the source interconnection 12A and the top surface of thegate lead-out electrode 6B. The final passivation film 13 is, forexample, a silicon oxide film deposited by a plasma chemical vapordeposition method which uses a tetraethoxysilane (TEOS) gas as aprincipal component of a source gas.

Then, a bonding opening in which the surface of a part of the sourceinterconnection 12A is exposed and a bonding opening in which thesurface of a part of the gate interconnection 12B is exposed are formedin the final passivation film 13. After that, the back of the n⁺-typesemiconductor substrate 1A is ground, and then the drain electrode 14 isformed on the back of the n⁺-type semiconductor substrate 1A as shown inFIG. 26. Thus, the power transistor having a MISFET having a trench gatestructure is almost finished.

As is apparent from the above description, similarly to thepreviously-described first embodiment, the fabrication method of thesecond embodiment comprises the steps of forming the trench.4 from themajor surface of the n⁻-type semiconductor layer 1B which serves as thedrain region, in the depth direction of the n⁻-type semiconductor layer1B, forming the gate insulating film 5 comprising the thermal oxide film5A and the deposition film 5B over the internal surface of the trench 4,forming the gate electrode 6A in the trench 4, introducing impuritiesinto the n⁻-type semiconductor layer 1B to form the p-type semiconductorregion 8 which serves as the channel forming region, and introducingimpurities into the p-type semiconductor region 8 to form the n⁺-typesemiconductor region 9 which serves as the source region. Accordingly,the second embodiment has effects similar to those of the firstembodiment.

Although the invention made by the present inventors has beenspecifically described with reference to the first and secondembodiments, the present invention is not limited to either of theaforesaid embodiments and various modification can of course be madewithout departing from the spirit and scope of the present invention.

For example, the present invention can be applied to a power transistor(semiconductor device) having a MISFET of p-channel conductivity typeand of trench gate structure.

Otherwise, the present invention can be applied to a power transistor(semiconductor device) having an IGBT (Insulated Gate BipolarTransistor) of trench gate structure.

The effect which representative aspects of the present inventiondisclosed herein have will be described in brief below.

It is possible to increase the reliability of a semiconductor devicehaving a transistor element of trench gate structure and provide stableFET characteristics of good reproducibility.

1. A method of fabricating a semiconductor device comprising the stepsof: (a) forming a trench in a semiconductor substrate having a firstsemiconductor region of a first conductivity type from a major surfaceof the semiconductor substrate in a depth direction thereof; (b) forminga gate insulating film including a thermal oxide film and a depositionfilm over the internal surface of the trench; (c) forming a gateelectrode over the gate insulating film in the trench; and (d)introducing, after said step (c), impurities into the semiconductorsubstrate to form a second semiconductor region of a second conductivitytype; and (e) introducing, after said step (c), impurities into thesemiconductor substrate to form a third semiconductor region of thefirst conductivity type; the third semiconductor region being formed ata place deeper than the first semiconductor region; the secondsemiconductor region being formed between the third semiconductor regionand the first semiconductor region in the depth direction. 2-21.(canceled)